The present invention relates to pumps in general, and more particularly to improvements in pumps which subject a fluid to and/or convey a fluid at very high pressures and/or temperatures. Typical examples of such pumps are primary recirculating pumps for pressurized water or boiling water reactors.
Pumps for use in nuclear power stations impose extremely strict requirements regarding the design, materials, manufacturing and quality assurance. such pumps operate at high system pressures and temperatures. The levels of radiation are high and, therefore, the pumps are confined in so-called containments which interfere with accessibility to the pumps, not only when the plant is in operation but also for servicing during the intervals of idleness. All this involves considerable expenses for special design and development of such pumps. Extraordinary measures are necessary to insure safety and reliability under all foreseeable circumstances and each unit must be tested extensively prior to delivery to the locale of use.
The first requirement which a nuclear reactor plant must satisfy is the provision of an enclosure, known as containment, which completely confines the pump, its motor, the pipes for circulating fluid and the steam generating vessel. This containment is to remain intact even in the event of a disaster of maximum forceable proportions. Such a disaster can arise in the unlikely event of complete breakage of a main coolant conveying pipe. Even a complete breakage of such pipe at right angles to its axis or a longitudinally extending crack with an area twice the cross-sectional area of the pipe must be controlled without fail, and the ability of the system to stand up under such disastrous circumstances must be proven to authorities beyond any reasonable doubt.
In a pressurized water reactor, the primary water is maintained at a pressure of up to and in excess of 150 bars and at a temperature of about 300.degree. C. A leak, e.g., the breakage of a main water pipe, results in the release of extremely large forces in the range of 1,000 megaponds (one pond is a unit of force equal to 9.80665 mg/s.sup.2). The forces which are released in the event of a disaster in a nuclear reactor plant will be more readily appreciated by considering that a reactor vessel (boiler) weighs about 500 megaponds and the pump and its motor together weigh between 70-100 megaponds, depending on their ratings. If the aforementioned forces act at right angles to the axis of a cracked or broken pipe, they generate bending stresses which are likely to result in plastic buckling. This can cause moments in the range of 3,000-5,000 Mpm, and such moments act on the pump which is normally welded into the piping. Moments in the range of 3-5 Mpm are normally considered as a very substantial stress upon the supports for the pump. Consequently, and in the absence of reliable precautionary measures, stresses of the above outlined magnitude are likely to rip one or more major components from the piping and the anchoring means therefor and to accelerate the separated components to the extent which is likely to result in penetration into and through the containment. Presently known devices which are used to prevent such separation of one or more major components employ shackles which are intended to hold the pipes, pump, motor and other components in place.
Another factor which must be considered in the design and anchoring of components of nuclear reactor plants is the possibility of earthquakes. The regulations governing the design of nuclear reactor plants provide that the plant must be capable of operating and/or of coming to a safe stop in the course of an earthquake. From the standpoint of vibration, a nuclear steam generating plant constitutes an elastically supported spatial multiple-mass oscillating system which, in the absence of preventive measures, would be likely to oscillate in synchronism with the waves developing in the course of an earthquake. In the event of vibrations caused by an earthquake, the heavier components of the system, such as the steam generators and the boiler, which are connected to each other by relatively small main coolant conveying pipes, would be likely to subject such pipes to extremely high stresses which could damage the pipes.
Attempts to counteract the consequences of tremors include such selection of characteristic frequencies of various components of a nuclear reactor plant that the frequency is well above the upper limit of the anticipated earthquake spectrum. Such measures also include the use of shackles which can perform their function only if they consist of extremely rigid ports. This cannot be readily achieved because the elements of such devices should not prevent heat expansion of the piping which is normally in the range of several centimeters.
The pump of a nuclear reactor plant is directly connected to the piping and is thus subjected to the stresses acting on the main components including the boiler and steam generator means. The lever action is likely to further enhance the effect of such stresses upon the pump. The radius of the pump is normally substantially less than the radius of a main component (e.g., 1 meter as compared with 2 meters in the case of the steam generator); therefore, the amplification of forces due to lever action upon the pump is very pronounced. If such stresses are to be resisted by anchoring means employing claws or the like, the claws must act on a relatively thin pump body. In certain instances, the stresses to which the housing of a pump is subjected by the anchoring means greatly exceed the pressure in the interior of the pump, and such stresses are not always localized but are likely to act upon a substantial or major part of the pump housing. Consequently, the housing of the pump must be designed with a view to withstand stresses which arise in normal operation as well as to withstand all such stresses which are likely to arise in the event of a major disaster including pronounced leakage and earthquakes. Indiscriminate strengthening of the pump housing for the purpose of considering such eventuality could result in the generation of excessive stresses due to changes in temperature, and such stresses could reach a value at which they would adversely affect the pumping action under normal operating conditions.